It is well known in the prior art to provide vehicles, such as farm tractors, with differentials which can be selectively locked or unlocked. It has also been long recognized that it is desirable to have the differential unlocked under certain conditions, as for example when applying the brakes, as the brakes are frequently employed when turning the tractor. One patent that teaches a control system for unlocking a selectively operable differential when the brakes are applied is U.S. Pat. No. 2,874,790 issued Feb. 24, 1959, to Hennessey. This patent further teaches that it is also desirable to unlock the differential when the ignition is turned off, or when turning. To accomplish the desired results an electric clutch unit is controlled by a plurality of series control switches. There are several disadvantages in this form of circuit. One disadvantage is that all leads and switches must be designed for carrying a current sufficient for the operation of the electric clutch unit. Another disadvantage of this form of circuit is that the contact switches of the current carrying type are relatively expensive and do not have the reliability as high as other components available today. A still further disadvantage of this design is that it is possible to start up the tractor when the differential is locked.
U.S. Pat. No. 3,706,351 issued Dec. 19, 1972, to Neisch and U.S. Pat. No. 3,845,671 issued Nov. 5, 1974, to Sharp et al both disclose the application of solid state logic elements to differential controls. However, neither of these patents disclose a system that permits the operator to control the differential while having certain circuits which override the operator controls to dispose the differential in an unlocked mode in certain conditions.
U.S. Pat. No. 3,732,752 to Louckes et al, issued in May, 1973, discloses the use of a manually operable pushbutton 66 to set an electromagnetic latch relay 68 for controlling a solenoid 46 and selectively locking a differential mechanism. A speed sensitive switch 62, in series with the latch, returns the differential to the unlocked state in high speed drive operation. However, no provision is made for the operator to disengage the latch, for the use of more reliable electronic latching components, or for automatic unlocking of the differential in the event of brake application of a power on condition.